Sense amplifiers have been widely used in the design of memory arrays in order to improve the speed of read operations. Conventionally, the memory cells in a memory array each have complementary outputs to indicate a positive and an inverted version of the one-bit value stored therein. Prior to each read operation, two bit lines are precharged high. Then, when a particular memory cell is selected by the address decoding circuitry of the memory array, the selected cell's complementary outputs are connected to the two precharged bit lines. Depending on the value stored in the selected cell, one or the other of the two bit lines will fall to ground. A sense amplifier having complementary outputs is used to detect the differential voltage that begins to develop between the two bit lines as one of them starts falling to ground. As soon as a sufficient voltage differential develops between the bit lines to trigger the sense amplifier, the contents of the selected memory cell will be reflected by the polarity of the sense amplifier outputs. The result is a dramatic improvement in the time required to perform read operations, because the sense amplifier need not wait until one of the bit lines falls all the way to ground before the contents of the memory cell are reliably indicated.
Unfortunately, such a scheme requires that two bit lines be routed across the memory array to the sense amplifiers for each access port. For this reason, a single-ended scheme that uses only a single bit line from the selected memory cells has become an attractive alternative design technique. When the single-ended scheme is used, the number of required bit lines in the memory array is reduced, as is the number of metal tracks that must be formed to implement the memory array. Often, this also reduces the area required to implement the memory array. In the single-ended scheme, the sense amplifier must compare the single bit line with a reference voltage. If the voltage on the bit line is lower than the reference voltage, the sense amplifier will indicate one value. If the voltage on the bit line is higher than the reference voltage, the sense amplifier will indicate an opposite value. Consequently, sense amplifiers have been designed that use sophisticated circuitry for creating and maintaining a reference voltage higher than ground but lower than V.sub.DD in an attempt to optimize performance in single-ended schemes. However, such sense amplifier circuits are complex and often result in reduced sensing margin.
A need therefore exists for a simple sense amplifier circuit, suitable for use in single-ended memory schemes, that does not require sophisticated circuitry for creating and maintaining a reference voltage.